Hepatitis C virus (HCV) infection is a major health problem that leads to chronic liver disease, such as cirrhosis and hepatocellular carcinoma, in a substantial number of infected individuals. Current treatments for HCV infection include immunotherapy with recombinant interferon-α alone or in combination with the nucleoside-analog ribavirin.
Several virally-encoded enzymes are putative targets for therapeutic intervention, including a metalloprotease (NS2-3), a serine protease (NS3, amino acid residues 1-180), a helicase (NS3, full length), an NS3 protease cofactor (NS4A), a membrane protein (NS4B), a zinc metalloprotein (NS5A) and an RNA-dependent RNA polymerase (NS5B).
One identified target for therapeutic intervention is HCV NS5A non-structural protein, which is described, for example, in Seng-Lai Tan & Michael G. Katze, How Hepatitis C Virus Counteracts the Interferon Response: The Jury Is Still Out on NS5A, 284 VIROLOGY 1-12 (2001); and in Kyu-Jin Park et al., Hepatitis C Virus NS5A Protein Modulates c-Jun N-terminal Kinase through Interaction with Tumor Necrosis Factor Receptor-associated Factor 2, 278(33) J. BIO. CHEM. 30711 (2003). A non-structural protein, NS5A is an essential component for viral replication and assembly. Mutations in NS5A at or near known sites of phosphorylation can affect the ability for high-level replication in cell-culture systems, suggesting an important role for NS5A phosphorylation in viral replication efficiency. Inhibitors of the phosphorylation of NS5A can lead to reduced viral RNA replication.
NS5A is a zinc metalloprotein organized into three discreet domains. NS5A localizes to the membrane-associated site of RNA synthesis via an N-terminal amphipathic α-helix anchor. The crystal structure of domain I demonstrates that NS5A can exist as a dimer, with a large putative RNA binding groove located at the interface of the monomers. Timothy L. Tellinghuisen et al., Structure of the zinc-binding domain of an essential component of the hepatitis C viral replicase, 435(7040) NATURE 374 (2005). Robert A. Love et al., Crystal Structure of a Novel Dimeric Form of NS5A Domain I Protein From Hepatitis C Virus, 89(3) J. VIROLOGY 4395-403 (2009). The interaction of NS5A with RNA is thought to be critical for the function of this protein in RNA replication. No structural information has yet been obtained for domains II or III. Recent genetic mapping has shown that although some residues in domain II are essential for RNA replication, many portions of domain II and all of domain III are dispensable. Timothy L. Tellinghuisen et al., Identification of Residues Required for RNA Replication in Domains II and III of the Hepatitis C Virus NS5A Protein, J. VIROLOGY 1073 (2008). Mutations constructed within domain III result in virus that can maintain RNA replication but that produces lower titers of infectious virus in cell culture, demonstrating a second distinct role for NS5A after RNA replication has occurred. Timothy L. Tellinghuisen et al., Regulation of Hepatitis C Virion Production via Phosphorylation of the NS5A Protein, 4(3) PLOS PATHOGENS e1000032 (2008); Nicole Appel et al., Mutational Analysis of Hepatitis C Virus Nonstructural Protein 5A: Potential Role of Differential Phosphorylation in RNA Replication and Identification of a Genetically Flexible Domain, 79(5) J. VIROLOGY 3187 (2005). NS5A, unlike the other non-structural proteins, can be trans-complemented, consistent with functions outside of the viral replicase. The interaction of NS5A with numerous host-signaling pathways has been described (Michael J. Gale Jr. et al., Evidence That Hepatitis C Virus Resistance to Interferon Is Mediated through Repression of the PKR Protein Kinase by the Nonstructural 5A Protein, 230 VIROLOGY 217 (1997); Andrew Macdonald & Mark Harris, Hepatitis C virus NS5A: tales of a promiscuous protein, 85 J. GEN. VIROLOGY 2485 (2004).), suggesting this protein may modify the host cell environment to a state favorable for the virus, events that may require a form of NS5A dissociated from the replication complex.
There is a clear and long-felt need to develop effective therapeutics for treatment of HCV infection. Specifically, there is a need to develop compounds that are useful for treating HCV-infected patients and compounds that selectively inhibit HCV viral replication.